Concentrations of carbon dioxide (CO₂) are increasing in the atmosphere, affecting soybean (Glycine max L.) phytohormone signaling and herbivore resistance. Whether the impact of elevated CO₂ on phytohormones and induced defenses is a generalized response within this species is an open question. We examined jasmonic acid (JA) and salicylic acid (SA) under ambient and elevated CO₂ concentrations with and without Japanese beetle (Popilliajaponica Newman) damage and artificial damage across six soybean cultivars (HS93–4118, Pana, IA 3010, Loda, LN97–15076, and Dwight). Elevated CO₂ reduced constitutive levels of JA and related transcripts in some but not all soybean cultivars. In contrast to the variation in JA, constitutive levels of salicylic were increased universally among soybean cultivars grown under elevated CO₂. Variation in hormonal signaling may underpin observed variation in the response of insect herbivores and pathogens to plants grown under elevated CO₂.